We propose to continue our investigations of four specific areas which relate to hypoxia in tumors. Three deal with overcoming the problem of the radioresistance of hypoxic cells in tumors, the fourth with exploiting hypoxia to achieve tumor-specific sensitization to chemotherapy and radiation. The four projects are: (1) In-depth studies of new radiosensitizers and other means of improving upon misonidazole (MIS) and its less toxic analog SR-2508. Included in this will be studies of depletion of glutathione (GSH) in vivo, and enhancement of sensitizer uptake by mild hyperthermia. (2) Studies of the possibility of enhancing hypoxic cell toxicity in tumors in vivo at clinical levels of radiosensitizers by depletion of intracellular GSH. (3) Investigations of acute and chronic hypoxia in a variety of experimental tumors including changing hematocrit levels and blood flow to improve tumor oxygenation. (4) Studies of the "preincubation" effect by which treatment of hypoxic cells with radiosensitizers renders them more sensitive to subsequent exposure to chemotherapeutic drugs and radiation. This will include a study of the mechanism of the enhancement of melphalan-induced DNA crosslinks, and the mechanism of the removal of the shoulder of the X-ray survival curve by pretreatment of cells to MIS. We will also investigate with CHO cells in vitro the potential clinical use of this shoulder removal, by determining whether it occurs in aerobic cells adjacent to hypoxic cells, whether it occurs at clinical doses and exposure times and whether it can be enhanced by GSH depletion, and finally, whether it occurs in tumors in vivo. If it can be produced in human tumors it would substantially increase their response to conventional radiotherapy. Four tumors will be used: the EMT-6, KHT and RIF-1 sarcomas and the SCC VII carcinoma. All will be assayed initially by in vivo-in vitro excision, and some by the regrowth delay and TCD50 assays. In Project 3 the effect of hematocrit and blood flow changes on the radiation response of out, skin and spermatagonia will also be tested. The mechanism studies in Project 4 will involve measurements of DNA strand-break and crosslink formation and repair using the alkaline elution assay.